Amino Acids PHC 211

 Characteristics and Structures of amino acids  Classification of Amino Acids  Essential and Nonessential Amino Acids  Levels of Protein Structure  Strecker amino acid synthesis  Lecture Contents:

Lectures Outcomes At the end of this lecture, students will be able to:  Explain some structural properties of amino acids.  Categorizing amino acids according to the nature of their side chains.  Illustrate the mechanism of Strecker amino acid synthesis.  Recognize four Levels of Protein Structure.

Amino Acids Characteristics and Structures

 There are 20 amino acids derived from proteins.  While there are several methods of categorizing them  One of the most common is to group them according to the nature of their side chains: 1- Nonpolar Side Chains 2- Polar, Uncharged Side Chains 3- Charged Side Chains Amino Acids

1- Nonpolar Side Chains There are eight amino acids with nonpolar side chains.  Glycine, alanine, and proline Have small, nonpolar side chains and are all weakly hydrophobic.  Phenylalanine, valine, leucine, isoleucine, tryptophan and methionine Have larger side chains and are more strongly hydrophobic. Amino Acids

2- Polar, Uncharged Side Chains  There are also eight amino acids with polar, uncharged side chains. Serine and threonine Have hydroxyl groups. Asparagine and glutamine Have amide groups. Cysteine Has a sulfhydryl group. Tyrosine Has a phenolic side chain. Amino Acids

3- Charged Side Chains There are four amino acids with charged side chains.  Aspartic acid and glutamic acid Have carboxyl groups on their side chains.  Arginine, lysine and Histidine Have side chains with amino groups. Amino Acids

Linear StructureAbbreviationName CH 3 -CH(NH 2 )-COOH ala A Alanine HN=C(NH 2 )-NH-(CH 2 ) 3 -CH(NH 2 )-COOH arg R Arginine H 2 N-CO-CH 2 -CH(NH 2 )-COOH asn N Asparagine HOOC-CH 2 -CH(NH 2 )-COOH asp D Aspartic Acid HS-CH 2 -CH(NH 2 )-COOH cys C Cysteine HOOC-(CH 2 ) 2 -CH(NH 2 )-COOH glu E Glutamic Acid H 2 N-CO-(CH 2 ) 2 -CH(NH 2 )-COOH gln Q Glutamine NH 2 -CH 2 -COOH gly G Glycine NH-CH=N-CH=C-CH 2 -CH(NH 2 )-COOH his H Histidine CH 3 -CH 2 -CH(CH 3 )-CH(NH 2 )-COOH ile I Isoleucine Amino Acids

Linear StructureAbbreviationName (CH 3 ) 2 -CH-CH 2 -CH(NH 2 )-COOH leu L Leucine H 2 N-(CH 2 ) 4 -CH(NH 2 )-COOH lys K Lysine CH 3 -S-(CH 2 ) 2 -CH(NH 2 )-COOH met M Methionine Ph-CH 2 -CH(NH2)-COOH phe F Phenylalanine NH-(CH 2 ) 3 -CH-COOH pro P Proline HO-CH 2 -CH(NH 2 )-COOH ser S Serine CH 3 -CH(OH)-CH(NH 2 )-COOH thr T Threonine Ph-NH-CH=C-CH 2 -CH(NH 2 )-COOH trp W Tryptophan HO-Ph-CH 2 -CH(NH 2 )-COOH tyr Y Tyrosine (CH 3 ) 2 -CH-CH(NH 2 )-COOH val V Valine Amino Acids

Essential and Nonessential Amino Acids  Nonessential amino acids are those that are synthesized by mammals,  while the essential amino acids must be obtained from dietary sources  The twenty amino acids can be divided into two groups of 10 amino acids.  Ten are essential and 10 are nonessential. However, this is really not accurate, as there is overlap between the two groups. Amino Acids

The Ten "Nonessential" Amino Acids Alanine Asparagine Aspartate Cysteine (requires sulfhydryl group from methionine) Glutamate Glutamine Glycine Proline Serine Tyrosine (synthesized from phenylalanine) The Ten "Essential" Amino Acids Arginine (see below) Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Amino Acids

 Amino acids are the building blocks of protein and you get protein from your diet.  If you eat a variety of healthy protein-containing foods, you will get all of the eight essential amino acids that your body can't make for itself.  Amino acids associate into small chains via chemical reactions called 'condensation reactions‘ in which the carboxyl group and the amino group bond to one another.  These small chains that contain a few amino acids linked together are called polypeptide chains (the individual amino acids are called peptides).  Polypeptide chains, in turn, combine into the more complex structures know as proteins Amino Acids

Levels of Protein Structure Structural features of proteins are usually described at four levels of complexity:  Primary structure: the linear arrangement of amino acids in a protein.  Secondary structure: folding or coiling within a protein; which are stabilized by hydrogen bonding.  Tertiary structure: the final three-dimensional structure of a protein, which results from a large number of non-covalent interactions between amino acids.

Cont. Levels of Protein Structure  Quaternary structure: non- covalent interactions that bind multiple polypeptides into a single, larger protein. Hemoglobin has quaternary structure due to association of two alpha globin and two beta globin polyproteins.  hemoglobin = 4 globins + 4 heme groups

Strecker amino acid synthesis Synthesis of α-amino acids by reaction of aldehydes with ammonia and hydrogen cyanide followed by hydrolysis of the resulting α-aminonitriles. Amino Acids

Reaction type: Nucleophilic Addition then Nucleophilic Acyl Substitution Summary  Reagents : NH 4 Cl / NaCN then aq. acid / heat with a basic work-up.  Nucleophilic cyanide ion normally adds to an aldehyde to give a cyanohydrin, an α-hydroxy nitrile.  However, in the presence of ammonium chloride, the analogous α-amino nitrile is obtained.  Remember that aldehydes are electrophilic at the carbonyl C atom.  Hydrolysis converts the nitrile to the carboxylic acid so providing the α-amino acid Amino Acids Cont. Strecker Synthesis

Amino Acids Cont. Strecker Synthesis Mechanism of the Reaction

Amino Acids Cont. Strecker Synthesis